So, we are assuming that Winston and FDR are killed?
Obviously, the USA and the UK are going to have to replace them. The USA puts the very leftist Charles Wallace as President, the UK going to have to appoint a new PM, could be Anthony Eden or Lord Halifax, or someone entirely different.
One CRITICAL concern is the USA is not at war with Germany, and they soon will be. Recall that FDR met Churchill BEFORE the USA's entry into WW2. FDR had wanted the USA to enter the war against Germany--and ironically, his own death might be the trigger for the war. The Declaration of War against Germany is a probable result of this attack, although it might be a more divided USA leading the attack.
This means that the USA goes to war five months earlier than before--predating Pearl Harbor. Hard to gauge how much more these five months are worth, but it favors the Allies. The USA is very, very, hot on invading Western Europe on their plans.
I expect that Japan will still attempt Pearl Harbor, but much of the US Navy has been withdrawn to the Atlantic for operations against Germany. Japan's advantage is minimized from the attack, and while it probably does gain a strong amount of territory from an largely undefended Pacific Ocean, this difference in power is directed against the Germans. We have a potential D-Day 1943 in the cards here. In any case, add 4 more months of wartime production against the Axis Powers.
In a democratic state, the political leaders of a country can only do so much in directing a war effort. Many of the UK's victories were in spite of, not because of, Churchill. A quieter leader might well allow the armed forces to do their jobs on their own. On the whole, I think this definitely favors the Allies, although butterflies might make the results confusing--one logical result is that Austria, Bohemia and Eastern Germany fall within the Allies' sphere of influence, changing the dynamics of the cold war.